A technique called MS/MS analysis (tandem analysis) is widely used as one method of mass spectrometry to identify substances with large molecular weights and to analyze the structures thereof. There are various configurations of mass spectrometers for performing MS/MS analysis, but one which has a comparatively simple structure and can be easily operated and handled is a tandem quadrupole (also called a triple quadrupole) mass spectrometer.
As described in Patent Literature 1 or the like, in a typical tandem quadrupole mass spectrometer, ions derived from a sample constituent produced by an ion source are introduced into a preliminary quadrupole mass filter (commonly described as Q1), and ions having a specific mass-to-charge ratio m/z are selected as precursor ions. These precursor ions are introduced into a collision cell internally equipped with a quadrupole (or greater number of poles) ion guide (commonly described as q2). A collision-induced dissociation (CID) gas such as argon is fed into the collision cell, and the precursor ions collide with the CID gas inside the collision cell and are split so that various product ions are produced. These product ions are introduced into a subsequent quadrupole mass filter (commonly described as Q3), whereby product ions having a specific mass-to-charge ratio m/z are selected and detected as they reach a detector.
Devices using a multistage dynode-type secondary electron multiplier or devices combining a conversion dynode, a fluorescent substance, and a photoelectron multiplier are used as detectors. In typical tandem quadrupole mass spectrometers of recent years, an analog detection signal obtained by such a detector is first sampled with a prescribed sampling period and then subjected to analog/digital (A/D) conversion, and the resulting digital data is summed (or averaged) over a prescribed duration of time (typically called the Dwell Time) so as to obtain measurement data for a given single point. For example, when performing MRM (Multiple Reaction Monitoring) measurements in GC/MS/MS or LC/MS/MS using a tandem quadrupole mass spectrometer as a detector of a gas chromatograph (GC) or a liquid chromatograph (LC), a mass chromatogram of the target mass-to-charge ratio is created using measurement data obtained by summation for each dwell time.
When performing MS/MS analysis with a tandem quadrupole mass spectrometer, product ions produced from ions selected by the preliminary quadrupole mass filter are detected after being selected by the subsequent quadrupole mass filter, so the arrival of undesired ions such as ions derived from impurities at the detector can be dramatically suppressed. Therefore, chemical noise, which is problematic in an ordinary mass spectrometer comprising only a single quadrupole mass filter, can be almost completely eliminated. However, since it is not possible to completely remove noise caused by the influx of neutral particles into the detector, spike-like noise is observed when the sensitivity (gain) of the detector is increased.
FIG. 4 (a) is an example of a mass chromatogram observed when MRM measurements are performed for 10 minutes with precursor ions of m/z=272 and product ions of m/z=241 in a state without a sample—that is, a state without a signal. As can be seen in the drawing, it can be confirmed that spike-like noise is generated almost randomly. Such noise becomes more prominent as the sensitivity of the detector is increased due to the nature of microanalysis, so the noise becomes a substantial impediment to the assay of trace constituents.
PATENT LITERATURE 1) Japanese Unexamined Patent Application Publication 2006-278024